The belt driven data tape cartridge of Von Behren, U.S. Pat. No. 3,692,255, has become a standard interface with computers where streaming and other off-line data back-up operations require rapid acceleration and deceleration of the tape. In these tape cartridges, a belt is driven by a drive roller along a belt path which includes a portion of the peripheries of the drive roller, a supply spool, a take-up spool, and two corner guide rollers positioned adjacent the tape spools, thereby reversibly driving the tape from spool to spool. A tape path extends between the spools and along one edge of the cartridge at which access to the tape and drive roller is provided.
The drive roller in the cartridge is externally driven by a drive puck in a cartridge drive, the periphery of which circumferentially contacts the periphery of the drive roller. The torque on the drive roller exerted by the cartridge drive's drive puck is a crucial operating parameter of a data cartridge. In order to assess the torque being applied, a data tape cartridge having a known desired internal resistance or "drag" can be inserted into a cartridge tester. Westlake Technology Corporation of Westlake Village, Calif., sells such cartridges and testers. The Westlake cartridge provides a drag by generating eddy currents within a rotating aluminum disk. This design is undesirable because the drag it provides is dependent on the temperature and speed of rotation of the aluminum disk. Furthermore, the aluminum disk protrudes out of the cartridge, thereby preventing insertion of the cartridge into a cartridge tester that has not been especially adapted to receive the cartridge. Furthermore, in the Westlake system, if the drive force measured by the cartridge tester for the verification cartridge does not match the actual drive force indicated on the verification cartridge, the entire cartridge tester must be returned to Westlake for re-calibration.